Loading system



Aug. 8, 1933. SHEA 1,921,431

LOADING SYSTEM Filed Aug. 11, 1930 2 Sheets-Sheet 2 FIG. 7 4 IKE LP/F HAMP LPflF AMPL IF/ER GAIN FREQUENCY -K/LOC ICLES HWEN TOR T.E SHE A BVJWPatented Aug. 8, 1933 A'r r o Flc LOADING SYSTEM Timothy E. Shea,Rutherford, N. .l., assignor to Bell Telephone Laboratories,Incorporated, New York, N. Y., aCorporation of New Yorlr ApplicationAugust 11, 1930. Serial No. 474,331

7 Claims. (o1. 17s

invention relates to loading systems for telep'-one lines and the like,and has for an object to improve the transmission and impedancecharacteristics of loaded lines.

This application is a continuation in part of the co-pending U. S.application of 'I. E. Shea,

Serial No. 198,459, filed June 13, 1927, now Patent No. 1,772,558 issuedAugust 12, 1930.

It is well known that the ordinary loaded line has an impedancecharacteristic which is a practically pure resistance but which variesquite considerably with frequency. This variation is particularly markedin the case of loaded cable circuits. On the other hand the impedance ofunloaded open-wire lines is practically independent of frequency so thatwhen an unloaded open-wire line is joined to an ordinary loaded lineconsiderable reflection loses are introduced;

In order to avoid such losses and also to simplify the impedancebalancing arrangementat repeaters, it is desirable that the loaded lineshould have an impedance which is resistive and practically constantwith frequency.

.Inone specific embodiment this invention comprises an open-wire lineor, cable periodically loaded by means of loading'unitscomprisinginductance coils and condensers connected in parallel witheach other in each side of the line. These loading units may bedistributed along the lines at approximately the same distances apart asare the loading coils in lines loaded by the Pupin method. Whenterminated mid-coil such system has a characteristic"impedance which isa substantially constant resistance throughout-the transmission range. 7

In a modified formthis invention comprises loaded system in which thesection or sections at each-terminal are loaded according to the methodof this invention as above described while the intermediate sections areloaded according to the ordinary method, i. e., the Pupin method. Itispossible to use this combination, because for frequencies in thetransmission range the mid-section characteristic impedance of a lineloaded with the loading units of this invention is substantially equalto the mid-section characteristic impedance ofa line loaded in the usualmanner.

In certain types of circuits, the loading netof this invention;

viding the improved characteristics mentioned above and in addition forreducing or prevent ing undesired cross-talk and distortion effects dueto electrostatic or electromagnetic induction between adjacent portionsof the circuits. One application of this'is in connection with loadedofiice entrance cable connecting a carrier repeater to an open-Wireline. Where the out.- put and input cable pairs of the entrance cableare in close proximity, as is usually the case, and the usual coilloading is applied, the electro static and electromagnetic inductionbetween the adjacent pairs-at high frequencies may be such as to producecirculating currents tending to cause regeneration and singing. If theloading networks of the invention are substituted for the usual loadingcoils in the entrance cable,

the sharply rising attenuation characteristics of the former above theloading cut-01f frequency erative effectsin the repeater.

@This invention will be better understood by reference to the followingdetailed description taken in connection with the accompanying drawingsin which: 7 7

Fig.1 shows a network substantially equivalent to a section of a cableor line inwhich the distributed series resistance and shunt capacity ofthe-line are represented by equivalent lumped impedances.

Fig. 2- illustrates the equivalent network-of Fig. 1 combined 'with'theusual'type of loading coils and terminated mid-coil; .Fig, 3 showsschematically a portion of a transmission lineloaded with-the loadingunits Fig. 4'shows a network equivalent to a single mid-coil'terminatedsection of the system of -Fig. 3; v

Fig. 5 shows the impedance-frequency characteristics of cables loaded inthe ordinary manner'and in accordance with the present invention; 7

Fig. 6 shows schematically a composite type of loaded line of thisinvention;

Fig. l'illustrates diagrammatically the usual arrangement of oiliceentrance cable associatm'. it)

ing two sections of an open-wire line with a two-way amplifyingrepeater; and,

Fig. 8 shows curves illustrating the application of the invention to asystem such as shown in Fig. 7.

If a section of cable or open-wire line in which the series resistanceand shunt capacity are the controlling factors, be represented by theequivalent T network shown in '1, where R is the total series resistanceof the section and S and C are respectively the length of the cablesection and the capacity for'each unit of length, so that the product isthe total ca pacity, such a section loaded in the usual manner may herepresented by the network shown in Fig. 2, where L is the totalinductance of the loading coil. In this case, as is well known,

the cut off frequency of the loaded sectionis approximately given by Thetwo most common methods of terminating such loaded sections are,respectively, mid-section and mid-coil. In the case of the midsectiontermination, the first loading coil is loand From which it is seen thatfor mid-coil and mid-l ,ction. to- "nations the characteristic impodanceis approximately a pure resistance, r

with the frequencies as a function the ratio of the frequency to thecut-off frequency.

Referring to Fig. 5 curve 13 represents the pedance-frequencycharacteristic of a cable 1 ded iii-the ordinary manner (Pupin method.)

th mid-coil termination, and asthe imped ce decreases with increasingfrequency, -apprcaching zero at the cut-off iiequency which for thiscase is about 2800 cycles per second. Curve A shows theimpedance-frequency characteristic for the'sainc type of cable withmidsection termination, as shown, in this the impedance increases withincreasing fref uency, approaching infinity at the cuteoff frequency. C*e C is the inpedance-frequcncy characteristlc of a non-loaded open-wireline that is,

characteristic, and its cut-oil frequency is ap proximately Il/L(SC+4C1) (4) Its nominal impedance is the same as for lines loadedinthe usual manner and is independent of the capacity of the seriescondensers. The characteristic impedance of a cable terminatedatmid-section with this typ loading is, for a given nominal impedance andgiven cut-off frequency, very nearly the same as that of the usual typeof loaded line, given by formula (3). While the variation of impedance.taken midsection, throughout the frequency hand is unaltered, this, ashereinafter explained, is advantageous rather than otherwise since itallows a cable of composite loading to be formed. The charcteristicimpedance of a cable tor inatcd at mid-load (mid-coil) -withthe antiresonant type of loading is, however, affected by the seriescondensers and isapproximately where f fthe anti-resonant frequency ofthe loading network, is

. 1 V g =2m/Ts1 Except where this anti-resonant frequency is very'clcsein value tothe cut-off frequency, the effect will he to make thecharacteristicim- LOO pedance very uniform throughout the transmissionband.

cmparing that for the two types of loading the mid-load impedancediffers by'the factor 1 T To and that this factor may be varied over awide range by a suitable choice of f that by a suitabi choice ofcapacity for the condensers.

In Fig. 5' curve D shows the impedance frequency characteristic for aloaded line of the type shown in Fig. 3 in which the anti-resonantfrequency is 1.25 times the cut-off frequency,

For a fixed value of loading coil inductance in each anti-resonantnetwork and fixed spacing ormulae) and 2), '1: is seen' quency. Thedilferencein transmission characteristics may be roughly expressedbysaying that the cut-off frequency is somewhat reducedas compared withthe ordinary type loading, although this effect may be minimized orprevented by reducing the loading spacing.

The attenuation and phase characteristics are not always controlling,however, and in certain cases it may be desirable, in an effort toobtain uniform impedance characteristics to sacrifice to some extent theobtaining of good attenuation and phase characteristics. Thisisparticularly true when toll entrance cable'or submarine cable must bejoined to open-Wire lines without'causing reflections at the junctionpoints. For example, in a case in which an entrance cable is used toconnect a repeater to an open-wire line, the repeater having a cut-offfrequency of about 2600 cycles per second and the nominal cut-offfrequency of the cable being fixed at 7200 cycles per second, only 36%of the transmission range is used. The impedance of the cable loaded inthe usual manner, as shown by curves A and B inFig. 5 will still beconsiderably difierentfrom that of the open-wireline, as shown by curveC. However, if the anti-resonant network in accordance with theinvention is substituted for the ordinary type of loading, and the samevalue of loading coil inductance and the same spacing are chosen as inthe case of ordinary loading sections and the condensers are chosen ofsuch capacity that f =L5fc the cut-off frequency of the cable would be5400 cycles and the impedance of the cable'would be practically the sameas that of the open-wire line throughout the entire effectivetransmission range. Also the attenuation and phase shift characteristicswould be practically identical with those of the cable loaded in theordinary Fig. 6 shows a composite type of loaded line in which some ofthe sections are loaded with anti-resonant networks and some are loadedin the ordinary manner. This arrangement is possible because themid-section characteristic impedence with either type of loading may bemade very closely the same by choosing the same cut-off frequencies andnominal impedances. The particular advantage .of this type of compositeloadedline is that the uniform impedance characteristics are desiredvprimarily at the ends'of the cable span, that is, at repeater andterminal points. By loading a few sections at either end of the cablespan with anti-resonant type of network the terminal impedance of thespan of cable may be made to closely approximate a constant value sothat it may be connected to terminal or repeater apparatus withoutreflection losses.

There may conceivably be cases where the attenuation characteristic ofthe line above the cut-off frequency is important in excluding in-.terfering frequencies induced from extraneous sources, and inasmuch asthe anti-resonant network has an attenuation characteristic which risessharply above the cut-off frequency, in the case of the correspondingtype of low pass filter, this proposed type of loading may be ofconsiderable value from this standpoint. Furthermore, the sections ofthis type when inserted in the cable, may have different frequencies ofanti-resonance, so that theattenuation ismaintained high for somedistance above the cut-off frequency. 1 Fig. -7 illustratesdiagramatically the usual arrangement of oflice entrance cableassociating an open-wire line with a carrier telephone repeater in amultiplex signal transmission system. The two-way repeater R may be forexample of the type disclosed in U S. Patent 1,413,357, to P. A.Raibourn, issued April 18, 1922. It comprises two one-way repeatingchannels RW and RE adapted to repeat signaling currentsbetween a westsection WL'and an east section EL of an open-wire line through entrancecable pairs ECl andECZ.

The signaling currents received at the repeater R over the cable pairECI from line section EL are amplified in the channel RW and theamplified currents are delivered to the line section WL over the cablepair EC2. Similarly the signaling currents received at therepeater Rover the cable pair E02 from the line section WL are amplified in thechannel RE and delivered to line section EL over cable pair ECi.

The currents incoming at the repeater from' the line section EL are ofhigher frequencies than those incoming thereat from the line section WL;Accordingly, the respective repeating channels are provided withfrequency discriminating filters which direct the incoming currents tothe proper channels. The repeating channel RW includes a'high pass inputfilter 1, a one-way amplifying device 2 and a I high pass output filter3. The repeating channel RE includes the low pass input filter 4, theone-way amplifying device 5 and the low pass output filter 6. Thedirectional filters 1 and 3 are designed to transmit currents oftheupper group of frequencies to be repeated by the amplifying device 2and to suppress from transmission currents of lower frequencies, whilethe directional filters 4 and 6 are designed to transmit currents of thelower group of frequencies to be repeated by the amplifying device 5 andto suppress from transmission currents of higher frequencies. Thefilters may be of the type disclosed in U. S. Patent No. 1,227,113. toCampbell, issued May 22, 1917.

The repeating paths RE and RW may contain other apparatus than thatshown, for. example, attenuation equalizers for compensating forunequalattenuation of the transmitted currents of different frequencies.

As the carrier repeater R ordinarily would be located at some distancefrom the mainpole leads of the open-wire line, it is the usual practiceto bring in the input and output wires of the repeater through entrancecable which is ordinarily loaded periodically with Pupinloading coils L,as indicated in the drawings, to reduce the attenuation to a'minimum.This coil load ing is usually designed to have a cut-off at a frequencywhich is well above the frequencies in the signaling range to betransmitted, in order to prevent reflections. For a range of signalingfrequencies up to 30 K. C. the cut-off frequency would be 50 K. 'C., forexample. On the other hand, in order to secure a fiat amplifiercharacteristic for the range of signaling frequencies up to 30 K. C. itmay be necessary to transmit fairly efficiently frequencies for aconsiderable distance above 30 K. C.

If the two entrance cable pairs EC1 and ECz are in close proximity, aswould commonly be the case, some electrostatic or electromagneticinduction will take place between them so that there will be a tendencyfor currents leaving the repeater at a high level to cross over to theopposite pair and reenter the repeater at a lower level. In this thereis a tendency towards regeneration or singing, due to the fact that acircuit may be formed by one side of the repeater, portions of theentrance cable pairs, and the leakage path therebetween.

For the current which travels through the low pass side of the repeater,that is, the channel there is little to worry about from thestandpointof circulating currents due to electrostatic orelectromagnetic induction, as the entrance cable pairs would beordinarily welltransposed with respect to each other. At highfrequencies, however, the transpositions become less effective and theelectrostatic and electromagnetic induction becomes greater.Consequently, it is the current which travels through the high pass sideof the repeater, that is, the channel RW (which will be high frequencycurrent) which may be troublesome.

The path of these high frequency circulatory currents may be traced asfollows. They will pass as indicated by the arrows from the east linesection EL through the cable pair E61 to the junction. pointA of therepeater and thence to the repeating channel RW. In the repeatingchannel RW these currents will pass through the high pass input filterl, and after being amplified by the amplifying evice 2 will be passedthrough the high pass output filter 3 to the point B of the repeater.From the point B these currents will pass t rough the loaded entrancecable pair ECz to the west line section WL, and in passing therethrough,will induce a corresponding high frequency current the adjacent entrancecable pair E 31, which will pass, as indicated by thev arrow, into therepeater at the junction point 'A, and will be amplified therein. It maybe readily seen that such regenerative and singing effects under certainconditions may be such as to prevent proper opera tion of the system. 1

The high side of the repeater will be effective in transmitting currentsto a much higherfreoucncy generally than the maximum conversationalfrequency. This maximum conversational frequency might be 30 K. C. andthe repeater might be efficient up to a frequency in the vici ity of 59ii. C. it would be possible to prevent currents of greater frequenciesthan 30 K. C. from flowing through this part of the repeaterby insertingtherein a separate low pass electrical filter. However, the same resultmay be accomplished by changing the type of loading and the entrancecable through which the circulating currents flow the sort ofcharacteristics that a low pass electrical filter would have. In otherwords, for circulating currents of freauencies between the maximumconversational frequency and the maximum frequency of transmission ofthe repeater, attenuaticns may be introduced to circulating currents atany point or points along their path. It may be introduced by a low passelectrical filter cutting off at a frequency of about 30 (3., or it maybe introduced by the use of properly designed loading networks in theentrance cable pairs.

If the loading networks of the invention either the anti-resonant typeshown in Fig. 3 or the composite type shown in 6, are substituted forthe coil loading networks L in the system of Fig. l, the sharply risingattenuation characteristic of these networks above the loading cut-oifrequencymay be utilizedto introduce considerable attenuation intransmitted frequencies in the range between 30 K. C'. and

K. C., that is, in the range occupied by the currents will take place ineach section of the loaded cable, and, for the induction takingplace inany particular section, the attenuation to which that particular portionof the circulating cu rents will be subjected, is that of one circuit ofthe entrance cable from the repeater down to the point where inductiontakes place plus the attenuation taking place in the induction processplus the attenuation in the corresponding portion of the entrance cablebetween the second point'of induction and the opposite side of there"eater. For-those elements of the circulating cu rents which arecaused by induction at points remote from the repeater, the attenuationwill therefore be high; forthose elements for whicinduction takes placeat a point close to the r peater the attenuation will be low. On theaverage, the elements of -circulating current subjected to aconsiderable amount of attenuation and consequently the circulatingcurrents willbe smaller with the anti-resonant type of loading or thecomposite type of loading or the invention than with the Pupin type ofloading, and the tendency toward regeneration or singing will beminimized.

The advantages which maybe obtained by the use oi loa" g networks of theinvention in the system or 7 will be clearer ircm an examination of thecharacteristic curves of "Fig. 8. In 8, the curve no representsthe fre"quency gain' characteristic of the amplif ing device 2 in the amplifyingpath RW; As indicated, it may have a fairly fiat gain characteristicover a frequency range extending from the lowest speech of about 58 K.C. Therefore. the amplifiertransmits some distance beyond themaximum'conversational frequency of say 30 K. C. The reason for this isgenerally inherent in amplifier and transformer des n. The curve APrepresents the attenuation equency characteristic for theloadingnetworks L in the system of .Fig. '1 corresponding to the wellknown Pupin loading coils, this characteristic being similar to t at ofa low pass electrical filter of corresponding structure. curvedesignated AAF. represents the attenuation frequency characteristic of"the.

anti-resonant loading network, such as shown in Fig. 3, where thefrequency of maximum attenuation corresponding to fm is selected bydesign. The curve Ac represents a combination of the anti-resonant andthe Pupin type of loading, such as shown "in Fig. 6, and gives anintermediate characteristic. Because this composite loading networkmaintains a fairly large attenuation at all frequencies a substantialdistance above the minimum conversational frequency it would often beused in preference to the anti-resonant type of network as shown in Fig.3. It may be seen from the curves of Fig. 8 that the characteristics A0and AAa rise sharply for frequencies immediately above 30 K. C. whilethe characteristic AP representing the .rupin type of loading rises muchmore gradually above 30 K. C. It may be seen therefore, that either ofthe loading networ s of the invention may be advantageously used inplace of the Pupin type loading network in the entrance cable in thesystem of Fig. 7 to minimize cross-talk effects as well as to obtainadditional frequencies up to frequency advantages from the standpoint ofimpedance constancy explained above.

In loading phantom circuits and side circuits in phantomed systems withthe anti-resonant type of loading network the condensers forming a partof the loading network in the side circuit produce no effect as far thephantom circuit is concerned, and similarly those used in the phantomcircuit produce no effect on the side circuits.

What is claimed is:

1. In a wave transmission system, twoadjacent two-way transmission'paths, means for impressing on said paths signal waves of one frequencyrange to be transmitted in one direction thereover, and signal waves ofa different frequency range to be transmitted in the opposite directionthereover, an amplifier for repeating the waves of said one frequencyrange over said paths in said one direction while substantiallyexcluding transmission of waves of said other frequency range, a secondamplifier for repeating the waves of said other frequency range oversaid paths in said opposite direction while substantially excludingtransmission of waves of said one frequency range, and means forperiodically,

loading said paths so as to obtain substantially uniform, lowattenuation in the transmitted waves of all frequencies in said rangeswhile producing high attenuation in transmitted waves of frequencies atleast immediately above the highest frequency in said ranges and therebyefiectively reducing undesired interchange of high frequency energy byinduction between said adjacent paths.

2. In a wave transmission system, two adjacent two-way transmissionpaths so arranged that there will be substantially no interchange of lowfrequency energy by induction therebetween, means for impressing on saidpaths, waves'of one frequency range to be transmitted in one directionthereover, and waves of a dif ferent frequency range to be transmittedthereover in the oppositedirection, two amplifiers for respectivelyrepeating waves of a different one of the two frequency ranges over saidpaths to the substantial exclusion of Waves of the other frequencyrange, and means for periodically loading said two-way paths to obtainsubstantially uniform low attenuation in transmitted Waves of allfrequencies in both ranges while producing high attenuation in wavefrequencies at least immediately above the highest frequency in saidranges.

3. The system of claim 1 and in which said loading means consists of aplurality of loading units connected in series in said paths, eachcomprising inductance and capacitance in parallel and having anattenuation characteristic which rises sharply above the loading cut-offfrequency.

4. The system of claim 1 and in which said loading means comprises aplurality of loading units connected in series with said paths andhaving sharply increasing attenuating properties in the frequency rangeabove the loading cut-off frequency.

5. The system of claim 1 and in which said loading means consists of aplurality of loading units connected periodically in series along saidpaths, each of said loading units comprising parallel inductance andcapacitance of such values as to make the unit anti-resonant at afrequency substantially greater than the loading cut-01f frequency, andto make the unit have a high attenuation for frequencies above theloading cut-off'frequency.

6. In combination, two sections of an openwire line and a two-wayrepeater interconnecting said sections, means for impressing on saidline signal waves of one frequency range to be transmitted thereover inone direction, and signal waves of a different frequency range to betransmitted thereover in the opposite direction, said repeatercomprising amplifying means for repeating res ectively waves of eachfrequency range between said line sections to the substantial exclusionof waves of the other frequency range, two twc-way cable pairsrespectively connecting said line sections to said repeater and in suchclose proximity as to allow'interchange of high frequency energytherebetween and means for periodically loading said cable pairs toobtain substantially uniform lcw attenuation in transranges. I

7. In combination, two sections of an openwire line and a two-wayrepeater interconnecting said sections, means for impressing on saidline signal waves of one frequency range to be transmitted thereover inone direction, and signal waves of a different frequency range to betransmitted thereover in the opposite direction, said repeatercomprising amplifying means for repeating respectively waves of eachfrequency range between said line sections to the substantial exclusionof waves of the other frequency range, two adjacent two-way transmissionpaths respectively connecting said line sections to said repeater, andarranged so that there is substantially no interchange of low frequencywave energy by induction between said paths, and means for periodicallyloading said paths to obtain substantially uniform low attenuation intransmitted waves of all frequencies in both ranges while producing highattenuation in transmitted waves of substantially all frequencies abovesaid ranges.

TIMOTHY E. SHEA.

